Livestock disease outbreaks are a serious threat to human and animal safety. For example, since 1999, there have been twenty-six avian influenza episodes involving twenty-five countries. These episodes required destroying of over 132 million birds and vaccinating an additional 1.9 billion birds. Avian influenza (AI) poses a significant threat to the United States poultry industry. One of the worst AI cases in recent U.S. history occurred in Pennsylvania in 1983, in which an outbreak of high pathogenic avian influenza (HPAI), H5N2, resulted in the destruction of 17 million birds at a cost of $61 million. In 2002, a low pathogenic avian influenza (LPAI), H7N2, outbreak in Virginia resulted in the destruction of 4.7 million birds at a cost of $160 million. The detection of two separate H7N2 LPAI strains in the Delmarva region in 2004 resulted in the destruction of 328,000 birds on two farms in Maryland and 85,000 birds on two farms in Delaware. A HPAI H5N2 outbreak in Gonzales County, Texas required depopulation of 6,608 broilers as a control measure.
Control of avian influenza consists of detection, confinement and depopulation or vaccination of the affected region. Quarantine of the area is an immediate step in control of an infectious poultry disease and can significantly reduce the spread of the disease. Birds that are infected or suspected of infection are depopulated using the most expedient method possible. Vaccination has been used in control of several outbreaks, however, the usefulness of conventional vaccination as an eradication tool remains unclear. Evidence indicates that the virus can continue to replicate in clinically healthy vaccinated birds, which reduces the effectiveness of vaccination programs. In addition, avian influenza is highly mutagenic, which can result in divergence between the vaccination strain and infection strain. For this reason, depopulation remains the primary control measures for avian influenza. Depopulation of poultry includes the euthanasia of the birds, removal of the carcasses and litter and decontamination or disinfecting of the facility and/or equipment.
Depopulation is also required to control outbreaks of other diseases. Exotic Newcastle Disease is an example of another avian disease where depopulation of infected birds and birds suspected of infection is a primary control measure.
Emergency depopulation of livestock is necessary under other conditions, such as natural disasters. Natural disasters, such as hurricanes, tornadoes, earthquakes, snow, wind, and floods, can damage livestock housing, leading to a need to quickly and efficiently depopulate livestock houses that are unsafe for human entry. For example, when poultry housing has been damaged such that food and water cannot be provided to the animals, the animals must be depopulated in a humane and safe manner.
Although a quick response is desired in the event of a disease outbreak, depopulation of birds must be performed in a humane manner. Depopulation and euthanasia describe similar applications. The American Veterinary Medical Association (AVMA) describes euthanasia as “rapid loss of consciousness followed by cardiac or respiratory arrest and the ultimate loss of brain function.” (2000 Report of the AVMA panel on euthanasia. JAVMA 218(5): 669-698.) The AVMA requires that the animal(s) to be euthanized experience minimal distress before being rendered unconscious. There are a number of currently accepted methods for euthanizing poultry include inhalation agents, non-penetrating captive bolt, cervical dislocation, percussive blow, decapitation, maceration, and electrocution. The methods cause death by (1) direct or indirect hypoxia, (2) direct depression of neurons and/or (3) physical disruption of brain activity. Extensive guidelines exist for euthanasia of poultry for processing prior to marketing. However, during emergencies such as a disease outbreak, there are fewer options. The 2000 Report of the AVMA Panel on Euthanasia, devotes only one paragraph on mass euthanasia and states “[u]nder unusual conditions, such as disease eradication and natural disasters, euthanasia options may be limited. In these situations, the most appropriate technique that minimizes human and animal health concerns must be used.”
Gassing is one of the accepted methods for euthanizing poultry. The gases most often employed include mixtures containing argon (Ar), nitrogen (N), carbon monoxide (CO), and/or carbon dioxide (CO2). Argon and nitrogen displace oxygen (O2) in the air, while CO2 directly affects the central nervous system.
Carbon dioxide is a well-known anesthetic gas that can induce rapid loss of consciousness, but at high concentrations (>65%) it is known to be an irritant to humans. Poultry are unable to detect the presence of CO2 before succumbing to the effects of the presence of high concentrations of the gas. The lack of reaction by the birds to the introduction of a gas falls within the suggested AVMA guidelines for the use of inhalants for euthanizing animals.
The AVMA has suggested that euthanasia is more humane if the animals are exposed to high concentrations of the inhalant so that unconsciousness is rapidly induced. In addition, the suggested exposure for optimal euthanasia is a five-minute period of exposure to CO2 in concentrations of 60 to 70 percent. Carbon dioxide stunning, however, is impractical as birds can regain consciousness in less than 30 seconds in some experiments.
Carbon dioxide-argon mixtures cause rapid loss of brain function in poultry. It has been suggested that moderate CO2 concentrations are more practical, particularly in on-farm applications. Emergency euthanasia, such as in the case of an AI outbreak, would need to be conducted on-site at the poultry farm to comply with quarantine regulations. Under field conditions, it is difficult to maintain a tightly governed range of gas concentrations.
Alternate gassing mixtures involving Ar or N rely on dilution of the oxygen in the surrounding atmosphere. Livestock houses are not airtight environments, and control of the environment is again a difficulty in those situations when emergency depopulation is necessary.
Several current industry procedures for large-scale emergency euthanasia consist of exposing poultry to CO2 gas while they are in the poultry house. Carbon dioxide gas causes rapid onset of anesthesia with subsequent death due to respiratory arrest. Portable panels with tarp covers were employed in the 2002 AI outbreak in Virginia. The panel enclosure was constructed inside the house and took 7 workers approximately 2 hours to construct per house. Groups of birds were driven and/or placed into theses units and euthanized with carbon dioxide. The panel enclosure method could be used for batches of up to 5,500 turkeys at a time and required between 6 to 10 minutes for audible signs of activity to cease.
A second procedure used for broilers in Virginia employed a metal container placed over a live-haul cage. The birds were caught by human catching crews and placed in a standard live-haul cage. After the cage was filled with birds, the cage was transported outside the building and a metal container was placed over the cage. Carbon dioxide gas was injected into the container. This method was also performed in a batch mode, with 375 chickens per batch, and required 1:00 to 1:40 (minutes:seconds) until audible signs of activity ceased.
A third procedure called the polyethylene tent method which has been used for broilers in Virginia and more recently on Delmarva. In the polyethylene tent procedure, the birds are condensed into one region of the house. Carbon dioxide cylinders are placed inside the euthanasia region and overlapping layers of polyethylene or other plastic sheeting are used the to cover birds. The polyethylene sheet is buried in the litter at the outside edge and overlapped in the region above the birds, forming a tent over the birds. After sealing the tent area, CO2 is released into the tent area.
All procedures are very labor intensive and create significant biosecurity risks. For example, the live-haul cage procedure requires the catching crew to be placed in intimate contact with birds infected with a potentially zoonotic virus. With this procedure, infected birds are transported outside the house prior to euthanasia, increasing potential spread. Euthanizing a typical farm with 75,000 to 100,000 market age broilers with the polyethylene tent method in one day would require a labor and support staff estimated in excess of 40 people. Furthermore, the process requires a tractor and additional personnel to remove the polyethylene from the houses. When this material is removed from the house, it may be heavily contaminated with litter, carcasses and debris, creating another biosecurity risk. Disposal of the contaminated polyethylene sheet via sanitary landfill or on-site burning creates yet another challenge for the procedure. While these procedures were effectively used to destroy flocks in the recent AI outbreaks in Delaware and Virginia, the current CO2 methods are very labor intensive and includes biosecurity hazards.
Accordingly, there is a need for an alternative method to depopulate livestock that minimizes labor resources and contamination, while maximizing efficiency, health, biosecurity, and safety.